1. Field of the Invention
This invention relates to a thermal transfer type printer or an impact type printer which feeds an ink sheet at 1/n times the feeding speed of a recording medium, (which is a so-called n times thermal transfer type printer) and especially to a thermal transfer type printer which controls the speed ratio between the feeding speed of a recording medium and a feeding speed of an ink sheet. (The n means a value which can be obtained by dividing the feeding speed of a recording paper by the feeding speed of an ink sheet, that is, n is a speed ratio between the feeding speed of a recording paper and the feeding speed of a ink sheet).
2. Description of the Prior Art
Generally, in a thermal transfer type printer, an ink sheet is placed on a recording paper and fed at a predetermined speed. A thermal transfer printer records an image on the recording paper by transferring ink to the recording paper using a thermal head to generate heat patterns in response to image data.
Recent developments in this area include an ink sheet which can be used with a number of sheets of recording paper. It is possible to record an image on the recording paper by feeding the ink sheet at 1/n times the speed of the recording paper, thereby decreasing the cost of using the ink sheet. The number n the "speed ratio", which is a ratio of the feeding speed of the recording paper and the feeding speed of the ink sheet.
The above described n times thermal transfer type printer can be constructed from a conventional thermal transfer type printer without having to make major modifications to the conventional thermal transfer type printer. The ink sheet might be too short.
FIG. 9 shows the relation between the printing density and the speed ratio n, which is determined by dividing the feeding speed of the recording medium by the feeding speed of the ink sheet.
As the value of the speed ratio n approaches 1 (or as the feeding speed of the recording medium approaches the feeding speed of the ink sheet, as shown in FIG. 9), the printing density becomes darker. Conversely, as the value of the speed ratio n increases (or as the feeding speed of the recording medium becomes greater than the feeding speed of the ink sheet, as shown FIG. 9), the printing density becomes lighter.
However, a problem with the n times thermal transfer type printer is that the thermal transfer type printer often does not use ink efficiently. That is, often there is ink remaining on the ink sheet after it is fed through the printer. This occurs because the feeding speed of the ink sheet used for recording an image having a light density or having no image data at all is the same speed that is used for recording an image of dark or maximum density.
In Japanese Laid Open Patent No. 2-22082, a thermal printer for solving the problem described above is disclosed.
The thermal transfer type printer efficiently consumes ink on an ink sheet by changing the feeding speed of the ink sheet according to the density level of image data in each line of the image.
The thermal transfer printer determines the speed ratio n (the ratio of the feeding speed of the recording paper to the feeding speed of the ink sheet) by comparing the maximum density of the line of the image with the maximum density of the image or by comparing average image density in each line with image data of maximum density. Accordingly, the ratios are determined based on the maximum data or the average data in each line of the image. The speed ratios can be adjusted by changing the feeding speed of the ink sheet line by line. The feeding speed of the recording paper is constant.
However, determining the speed ratio line by line takes much time. Moreover, it can be difficult to change the feeding speed of the ink sheet line by line because the feeding speed of the ink sheet is controlled by a motor which may delay in responding to the change.
Further, since Japanese Laid Open Patent No. 2-22082 discloses only a thermal printer which uses a mono color ink sheet and does not discuss using a length of color ink area. The mono color thermal printer technology can not be applied to a thermal transfer printer which uses an ink sheet having repeatedly arranged color ink areas because lengths of these ink areas are predetermined. A recording area can exceed one color ink area so that a desired color image is not obtained.